1. This invention relates to titanium base alloys of the Ti.sub.3 Al (alpha two) type which are usable at elevated temperatures and have useful ductility at lower temperatures.
2. Titanium alloys have found wide use in gas turbines in recent years but they are limited in use to temperatures below 600.degree. C. by decreasing strength. During the last twenty years there was considerable work on higher temperature alloys particularly those derived from the ordered alloys Ti.sub.3 Al (alpha two phase) and TiAl (gamma phase). However, none of the prior alloys based on TiAl and Ti.sub.3 Al has been found useful in engineering applications, mostly because the alloys which had strength did not have adequate low temperature ductility. Other factors limiting alloys' utility are lack of metallurgical stability, high density and lack of fabricability, (ability to be cast, forged, machined, etc.).
Presently, iron, nickel, and cobalt super alloys are used at temperatures beyond those at which titanium alloys are able to perform. To replace such alloys, of which nickel alloy INCO 713C is an example, new titanium alloys must have equal or better strength to density ratios. To be useful as engineering materials they also must have ductility at room and intermediate temperatures; that is, desirably at least 1.5% tensile elongation at room temperature and around 3% at 200.degree.-400.degree. C.
There has been further research on titanium aluminide alloys in the last few years, and this, coupled with improved tools and knowledge of metallurgy, has now produced new advances. In our copending application Ser. No. 060,265, filed July 25, 1979, we described new alloys of the TiAl type. We are now also able to disclose herein new alloys of the Ti.sub.3 Al type. It is well appreciated by those skilled in the art that the two alloy types are quite metallurgically distinct and have dissimilar alloying characteristics (as in fact, a comparison of our applications will support).
For much background on the prior art in titanium aluminum alloys, we make reference, and incorporate, the Background in our aforementioned application. Of the references in our other application, Jaffee U.S. Pat. No. 2,880,087 is worth further note here. Ti.sub.3 Al in weight percent is the alloy Ti-14AL1. Jaffee broadly discloses alloys of 8-34 weight percent aluminum containing from 0.5-50 percent columbium, vanadium, many other elements, and mixtures thereof, but no teaching is given on proportions of the elements V and Cb, nor of any particular criticality within the range. It will be seen below that such broadly comprised alloys are not of utility in engineered machines.
Winter in U.S. Pat. No. 3,411,901 discloses Ti-Al-Nb alloys, particularly those having by weight percent 10-30 Al, and 8 parts Nb for every 7 parts Al. Specific alloys range from Ti-12Al-12Nb to Ti-17.5Al-20 Nb. The alloy compositions taught by Winter are constrained, as the phase diagram, FIG. 1, of his patent indicates. The alloys fall along the line which includes the compositions TiNbAl.sub.3 and NbAl.sub.3, and define the particular relation of Nb and Al, which we have now discovered does not produce the best properties. While Winter discloses favorable 800.degree. C. tensile elongations of about 5.15%, lower temperature ductilities are not disclosed. Additions of Si, Hf, Zr, and Sn are mentioned to improve workability and strength.
In the early 1960's McAndrew et al made reports entitled "Investigation of the Ti-Al-Cb System as a Source of Alloys for Use at 1200.degree.-1800.degree. F.". Among these reports are WADD 60-99 and ASD-TR-61-446, Parts I and II, published by the U.S. Air Force, Wright Paterson Air Force Base, Ohio. Initially a matrix of alloys was cast, containing by weight 5-15% Al and from 15-30% Cb in increments of 2.5%. The strong effect of Al was noted in all Cb contents, although this is not to say it was entirely consistent. In the second phase, sheet was made from scaled up heats of Ti-15Al-17.5Cb and Ti-10Al-15Cb to evaluate heat treat response and other behavior. Since none of the Ti-Al-Cb alloys were deemed to have adequate combination of properties, subsequent work evaluated improved purity (no strong effect found) and additions of 1-5% Zr, Hf and Sn. It was concluded that alloys of high Cb and Al content were preferred with quarternary additions of Hf and Zr. Also seen to be promising were Ti-12.5/15Al-22.5Cb-0.5/5(Hf/Zr/Sn). The third and final phase of the work included evaluation of Ti-12.5Al-35Cb and Ti-17.5Al-17.5Cb; but these alloys had negligible room temperature ductility. The most promising alloys were seen to be Ti-13Al-25Cb-5Hf-0.1C and Ti-15Al-22.5Cb-1Sn. Heat treatments and other processing were also reported on. Although still appearing foresighted in systematic pursuit of the Ti-Al-Cb system, McAndrew et al did not succeed in establishing for the Ti-Al-Cb system the optimum relationship of Al and Cb, although some of their test alloys came near to those which we will reveal below. The teaching of the McAndrew et al work is that there is no particularly promising Ti-Al-Cb alloy except those which contain 1-5% Hf/Zr/Sn. And of Ti-Al-Cb-Hf/Zr/Sn alloys, the teaching from the two aforementioned least unpromising alloys is that when Al is increased, Cb should be decreased.
Thus, it may be said first that the prior art reveals Ti-Al-Nb alloys in general and certain specific compositions. Among the various beta promoters there is no strong distinction especially insofar as providing advantage in a combination of low temperature ductility and creep resistance.